Human aging is associated with declining bone mass (e.g. in osteoporosis). Bone-forming osteoblasts are derived from multipotent human mesenchymal progenitor cells (hMPCs). Cell senescence is an irreversible proliferation arrest that prevents unlimited rounds of cell division. Shortened chromosome telomeres, resulting from multiple rounds of cell proliferation, are one trigger of senescence. By arresting cell proliferation, senescence is thought to prevent indefinite tissue renewal, and so contribute to tissue aging. Also, cell senescence inhibits osteogenesis (bone differentiation) in vitro. In sum, senescence may disrupt cycles of adult bone resorption and deposition, contributing to declining bone mass with age. Recently, my lab has shown that a complex of histone chaperones, including HIRA and ASFla, is responsible for remodeling chromatin structure in senescent cells. Our recent data indicate that the HIRA/ASFla pathway contributes to both senescence and osteogenic differentiation of hMPCs. We will investigate the role of the HIRA/ASF la pathway in differentiation of hMPCs along the osteogenic lineage and its contribution to bone formation in vivo, in addition, we will ask how senescence inhibits osteogenesis in vitro, whether senescent mesenchymal cells accumulate in prematurely-aged osteoporotic bone and assess the impact of HIRA/ASFla on this osteoporotic phenotype. We will also address the osteogenic function of a new-found member of the HIRA/ASFla complex, a protein called Ubnl. In collaboration with the other projects, this project will: utilize a novel X-ray structure of the HIRA/Ubnl histone chaperone complex, generated in Project 1, to dissect the role of that complex in osteogenic differentiation;interact with Project 2 through functional analysis of the yeast orthologs of HIRA and Ubnl; and, in collaboration with Project 3, investigate the contribution of short telomere-induced cellular senescence to age-dependent osteoporosis in the mouse, and the impact of HIRA and ASFla on this. In sum, Project 4 rounds off the program project, and applies the structural and genetic discoveries from Projects 1-3 to an aspect of mammalian tissue aging.